Silicon wafers are cleaned by exposure to aqueous chemical solutions prior to each thermal exposure step, i.e., between 8 and 16 times during the fabrication of a typical integrated circuit device. It is customary in the semiconductor industry to dry silicon wafers by spinning them in a centrifuge. This has a number of disadvantages. The material of construction of the carrier cannot be quartz because quartz is not strong enough, and this implies that the wafers must be cleaned and dried in a plastic carrier and then transferred to quartz (which in turn has to be cleaned separately) before annealing, diffusion or oxidation. Another problem is that centrifuging is essentially a batch operation and does not fit in well with continuous processing. Also, the downtime of a centrifuge is relatively great because, like all highly stressed mechanical systems, a centrifuge requires maintenance and, besides, is difficult to clean if it gets contaminated by insertion of an improperly handled, dirty carrier, by the inevitable breakage of a wafer that showers debris, or by particle-laden air being dragged into the chamber.
Another type of drying system in current use relies on a high-speed hot-air dryer to first remove all the large liquid drops of rinse water from the surface, then raise the temperature to about 100.degree. C. to insure complete removal. This takes almost 3 minutes and requires the input of about 6 kW electrical heat for heating the filtered air needed to dry one carrier with twenty-five 76-mm-diameter wafers, plus an airflow of several hundred cubic feet per minute. With the invention of megasonic cleaning (see U.S. Pat. No. 3,893,869 issued to A. Mayer and S. Shwartzman on July 8, 1975, and assigned to RCA Corporation), hot-air drying offers many advantages over spin drying and in particular permits the cleaning and drying operation to be carried out in the same carrier as the subsequent thermal treatment, such as oxidation, diffusion, or annealing. One of the advantages of hot-air drying, in comparison with centrifuging, is that the liquid is not evenly spread over the surface of the wafers. Thus, if streaks are visible, it is immediately clear that the water supply is dirty and needs attention; conversely, if the water used in rinsing is clean, no streaks develop. This self-indicating feature is most valuable in detecting a problem long before device electrical tests would show that it exists. The best utilization achieved so far is the drying within five minutes of the contents of two carriers, one stacked over the other and each holding twenty-five wafers. Conceivably, the number of wafers could be doubled by closer spacing, but even so hot-air drying would require about 5 Wh/wafer plus the cost of the high-velocity air.
The present invention comprises a novel gas drying system designed to reduce the cost of cleaning silicon wafers, which would be an important step in making the production of silicon solar cells more economical. The invention to be described shows that it is unnecessary to heat the gas stream and that drying can be achieved in the same time of 0.5 to 2 minutes with roughly the same gas flow of 200 to 800 cubic feet per minute depending on the carrier geometry, for 3 inch (7.62 cm) diameter wafers.